Move bounds checks on copyto!(dst, n, src) (#43517)

Author: mcabbott

base/abstractarray.jl

@@ -914,9 +914,17 @@ end
 
 function copyto!(dest::AbstractArray, dstart::Integer, src)
     i = Int(dstart)
-    for x in src
-        dest[i] = x
-        i += 1
+    if haslength(src) && length(dest) > 0
+        @boundscheck checkbounds(dest, i:(i + length(src) - 1))
+        for x in src
+            @inbounds dest[i] = x
+            i += 1
+        end
+    else
+        for x in src
+            dest[i] = x
+            i += 1
+        end
     end
     return dest
 end

test/arrayops.jl

@@ -2105,6 +2105,16 @@ end
     @test_throws ArgumentError LinearAlgebra.copy_transpose!(a,2:3,1:3,b,1:5,2:7)
 end
 
+@testset "empty copyto!" begin
+    @test isempty(copyto!(Int[], ()))
+    @test isempty(copyto!(Int[], Int[]))
+    @test copyto!([1,2], ()) == [1,2]
+
+    @test isempty(copyto!(Int[], 1, ()))
+    @test isempty(copyto!(Int[], 1, Int[]))
+    @test copyto!([1,2], 1, ()) == [1,2]
+end
+
 module RetTypeDecl
     using Test
     import Base: +, *, broadcast, convert